1. Field of the Invention
The present invention relates to an autotensioner used in a belt system for transmitting drive power of, for example, an automobile engine to a driven pulley by a transmission belt.
2. Description of the Related Art
Conventionally, there is known an autotensioner, which is provided in a driven apparatus for transmitting drive power of an automobile engine to a plurality of equipments through a transmission belt, to reliably transmit the drive power to each of the equipments by imparting tension to the transmission belt. Such an autotensioner is provided with a stationary member so that it can be fixed to an engine block, for example, an arm rocking with respect to the stationary member, and a pulley attached rotatably to the arm. A torsion coil spring, for example, is housed in the stationary member so as to give tension to the transmission belt through the pulley.
In such an autotensioner, when the transmission belt vibrates, the arm rocks and a load acts between the arm and stationary member. To counter this load and attenuate the vibration of the belt and to prevent damage caused by contact between the arm and the stationary member, a friction member formed from a synthetic resin, for example, is fixed to the arm, and slides against the stationary member when the arm rocks. For the engagement of the friction member, it is known to use a C-spring biasing the friction member from the inside thereof by a substantially constant pressure. For example, this configuration is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-338487.
However, a C-spring has to be set in material and shape in accordance with the required pressure. Further, it is necessary to provide a structure for engaging the C-spring with the friction member. Thus, when using a C-spring, there are the problems of a complicated configuration and increased manufacturing cost.
Therefore, an object of the present invention is to provide an autotensioner in which a friction member is fixed by a simple structure without using a C-spring to generate the required damping force.
According to the present invention, there is provided a cup-shaped stationary member, an arm, a pulley, and a first friction member.
The cup-shaped stationary member has an opening and a bottom, in which an axial bore is formed. The arm is attached to the opening. The arm has a rocking shaft, which extends to the bottom and is inserted into the axial bore, so that the arm rocks about the rocking shaft. The arm has a stub shaft offset from the rocking shaft and extending in the opposite direction to the rocking shaft. The pulley rotates about the stub shaft and gives a tension to a transmission belt. The first friction member is provided between an annular wall of the stationary member, which is positioned close to the opening, and a rocking wall formed on the arm, to generate a first frictional resistance by rocking of the arm.
By the simple structure in which the friction member is gripped between the circumferential wall and the rocking wall, a damping force is generated.
The autotensioner may be provided with a second friction member interposed between the axial bore and the rocking shaft to generate a second frictional resistance by rocking of the arm. By providing this second friction member, along with the first friction member, the rocking of the arm is attenuated.
Preferably, the first friction member has a friction surface generating the first frictional resistance with the rocking wall by rocking of the arm, and the area of the friction surface is set to a size in accordance with a maximum load acting on the first friction member.
The area of the friction surface of the first friction member may be determined by the following formula:
A={(a+b)/a}xc3x97F/P
wherein A is the area of the friction surface of the first friction member, a is the distance from a first peak position where a maximum load acts on the second friction member to a second peak position where a maximum load acts on the first friction member, b is the distance from the second peak position to a third peak position where a maximum load acts on the pulley, F is a maximum load acting on the pulley, and P is a withstand pressure of the first friction member.
Preferably, the first friction member is made of a synthetic resin mainly comprised of polyphenyl sulfone, and the synthetic resin exhibits a limited PV factor substantially exceeding 2.0 MPaxc2x7m/sec when sliding against the arm at a speed of substantially 0.5 m/sec. By making the first friction member of a material with a high limited PV factor, a sufficient durability can be exhibited against rocking of the arm.
The rocking wall and the annular wall may face each other and be substantially parallel, and the first friction member may have a bearing portion formed in a tubular shape between the rocking wall and the annular wall. Such a first friction member is easy to form.
The rocking wall may face the annular wall at a slant, and the first friction member may have a bearing portion formed in a taper between the rocking wall and the annular wall. Such a first friction member can exhibit a high durability with respect to the radial load by adjusting the thickness of the shaft member in accordance with the distribution of the load acting on the bearing portion.
Preferably, a normal-rotation damping force, acting on the arm when the arm moves in a first direction in which the transmission belt slacks, is greater than a reverse-rotation damping force, acting on the arm when the arm moves in a second direction in which the transmission belt is tensioned.
Further, preferably, a dynamic damping force acting on the arm is greater than a static damping force acting on the arm. In this case, the dynamic damping force is more than two times the static damping force.